Introducing a maize gene into switchgrass substantially boosted the potential of the switchgrass biomass as an advanced biofuel feedstock. Photo: USDA/ARS |
Many experts believe that advanced biofuels made
from cellulosic biomass are the most promising alternative to petroleum-based
liquid fuels for a renewable, clean, green, domestic source of transportation
energy. Nature, however, does not make it easy. Unlike the starch sugars in
grains, the complex polysaccharides in the cellulose of plant cell walls are
locked within a tough woody material called lignin. For advanced biofuels to be
economically competitive, scientists must find inexpensive ways to release
these polysaccharides from their bindings and reduce them to fermentable sugars
that can be synthesized into fuels.
An important step towards achieving this goal has
been taken by researchers with the U.S. Department of Energy (DOE)’s Joint
BioEnergy Institute (JBEI), a DOE
Bioenergy Research
Center led by the
Lawrence Berkeley National Laboratory (Berkeley Lab).
A team of JBEI researchers, working with
researchers at the U.S. Department of Agriculture’s Agricultural Research
Service (ARS), has demonstrated that introducing a maize (corn) gene into
switchgrass, a highly touted potential feedstock for advanced biofuels, more
than doubles (250%) the amount of starch in the plant’s cell walls and makes it
much easier to extract polysaccharides and convert them into fermentable
sugars. The gene, a variant of the maize gene known as Corngrass1 (Cg1), holds
the switchgrass in the juvenile phase of development, preventing it from
advancing to the adult phase.
“We show that Cg1 switchgrass biomass is easier for
enzymes to break down and also releases more glucose during saccharification,”
says Blake Simmons, a chemical engineer who heads JBEI’s Deconstruction
Division and was one of the principal investigators for this research. “Cg1
switchgrass contains decreased amounts of lignin and increased levels of
glucose and other sugars compared with wild switchgrass, which enhances the
plant’s potential as a feedstock for advanced biofuels.”
The results of this research are described in a
paper published in the Proceedings of the National Academy of Sciences (PNAS)
titled “Overexpression of the maize Corngrass1 microRNA prevents flowering,
improves digestibility, and increases starch content of switchgrass.”
JBEI researchers studying Cg1 switchgrass included (foreground) Seema Singh, (from left) Chenlin Li, Lan Sun, Blake Simmons, and Dean Dibble. Photo: Roy Kaltschmidt, Berkeley Lab |
Lignocellulosic biomass is the most abundant
organic material on earth. Studies have consistently shown that biofuels
derived from lignocellulosic biomass could be produced in the United States
in a sustainable fashion and could replace today’s gasoline, diesel and jet
fuels on a gallon-for-gallon basis. Unlike ethanol made from grains, such fuels
could be used in today’s engines and infrastructures and would be
carbon-neutral, meaning the use of these fuels would not exacerbate global climate
change. Among potential crop feedstocks for advanced biofuels, switchgrass
offers a number of advantages. As a perennial grass that is both salt- and
drought-tolerant, switchgrass can flourish on marginal cropland, does not
compete with food crops, and requires little fertilization. A key to its use in
biofuels is making it more digestible to fermentation microbes.
“The original Cg1 was isolated in maize about 80
years ago. We cloned the gene in 2007 and engineered it into other plants,
including switchgrass, so that these plants would replicate what was found in
maize,” says George Chuck, lead author of the PNAS paper and a plant molecular geneticist who holds joint
appointments at the Plant Gene Expression Center with ARS and the University of
California (UC) Berkeley. “The natural function of Cg1 is to hold pants in the
juvenile phase of development for a short time to induce more branching. Our
Cg1 variant is special because it is always turned on, which means the plants
always think they are juveniles.”
Chuck and his colleague Sarah Hake, another
co-author of the PNAS paper and
director of the Plant
Gene Expression
Center, proposed that
since juvenile biomass is less lignified, it should be easier to break down
into fermentable sugars. Also, since juvenile plants don’t make seed, more
starch should be available for making biofuels. To test this hypothesis, they
collaborated with Simmons and his colleagues at JBEI to determine the impact of
introducing the Cg1 gene into switchgrass.
In addition to reducing the lignin and boosting the
amount of starch in the switchgrass, the introduction and overexpression of the
maize Cg1 gene also prevented the switchgrass from flowering even after more
than two years of growth, an unexpected but advantageous result.
“The lack of flowering limits the risk of the
genetically modified switchgrass from spreading genes into the wild
population,” says Chuck.
The results of this research offer a promising new
approach for the improvement of dedicated bioenergy crops, but there are
questions to be answered. For example, the Cg1 switchgrass biomass still
required a pre-treatment to efficiently liberate fermentable sugars.
“The alteration of the switchgrass does allow us to
use less energy in our pre-treatments to achieve high sugar yields as compared
to the energy required to convert the wild type plants,” Simmons says. “The
results of this research set the stage for an expanded suite of pretreatment
and saccharification approaches at JBEI and elsewhere that will be used to
generate hydrolysates for characterization and fuel production.”
Another question to be answered pertains to the
mechanism by which Cg1 is able to keep switchgrass and other plants in the
juvenile phase.
“We know that Cg1 is controlling an entire family
of transcription factor genes,” Chuck says, “but we have no idea how these
genes function in the context of plant aging. It will probably take a few years
to figure this out.”